1. Field of the Disclosure
The disclosure relates to an organic light emitting diode (OLED) display device, which may be referred to as an organic electroluminescent display device, and more particularly, to an OLED display device having improved aperture ratio and lifetime.
2. Discussion of the Related Art
An OLED display device of new flat panel display devices has high brightness and low driving voltage. The OLED display device is a self-emitting type and has excellent view angle characteristics, contrast ratio, a response time, etc.
Accordingly, the OLED display device is widely used for a television, a monitor, a mobile phone, etc.
The OLED display device includes an array element and an organic light emitting diode. The array element includes a switching thin film transistor (TFT), which is connected to a gate line and a data line, a driving TFT, which is connected to the switching TFT, and a power line, which is connected to the driving TFT. The organic light emitting diode includes a first electrode, which is connected to the driving TFT, and further includes an organic emitting layer and a second electrode.
In the OLED display device, light from the organic emitting layer passes through the first electrode or the second electrode to display an image. A top emission type OLED display device, where the light passes through the second electrode, has an advantage in an aperture ratio.
Generally, the organic emitting layer is formed by a thermal deposition method using a shadow mask. However, the shadow mask sags because the shadow mask becomes larger with an increase in sizes of display devices. As a result, there is a problem in deposition uniformity in the larger display device. In addition, since a shadow effect is generated in the thermal deposition method using the shadow mask, it is very difficult to fabricate a high resolution OLED display device, e.g., above 250 PPI (pixels per inch).
Accordingly, a new method instead of the thermal deposition method using the shadow mask has been introduced.
In the new method, a liquid phase organic emitting material is sprayed or dropped in a region surrounded by a wall using an ink-jet apparatus or a nozzle-coating apparatus and cured to form the organic emitting layer.
FIG. 1 is a schematic cross-sectional view showing an OLED display device in a step of forming an organic emitting layer by spraying or dropping a liquid phase organic emitting material.
To spray or drop the liquid phase organic emitting material by the ink-jet apparatus or the nozzle-coating apparatus, a bank 53, which is formed on the first electrode 50 and surrounds a pixel region P, is required to prevent the liquid phase organic emitting material from flooding into a next pixel region P. Accordingly, as shown in FIG. 1, the bank 53 is formed on edges of the first electrode 50 before forming the organic emitting layer 55.
The bank 53 is formed of an organic material having a hydrophobic property. The hydrophobic bank 53 prevents the organic emitting material, which has a hydrophilic property, from being formed on the bank 53 and flooding into a next pixel region P.
By spraying or dropping the liquid phase organic emitting material from a head of the ink-jet apparatus or a nozzle of the nozzle-coating apparatus into the pixel region P, which is surrounded by the bank 53, the pixel region P is filled with the organic emitting material. The organic emitting material is dried and cured by heat to form the organic emitting layer 55.
However, the organic emitting layer 55 has a difference in a thickness. Namely, the organic emitting layer 55 has a thickness in edges being larger than a thickness in a center.
If the organic emitting layer 55 has a thickness difference, the OLED display device has a difference in emitting efficiency. Accordingly, as shown in FIG. 2, which is a picture showing one pixel region in the related art OLED display device, dark images are displayed in edges of the pixel region. In this instance, since the dark images are perceived as an image defect by the viewer, the edges of the pixel region should be shielded such that the edges of the pixel region do not serve as an effective emission area.
Referring again to FIG. 1, an effective emission area EA1 is a portion of the pixel region P where the organic emitting layer 55 has a flat top surface. Namely, the aperture ratio of the OLED display device is decreased.